Genetic Tumors in Nicotiana

Studies were made of thirty-one interspecific tumor-forming hybrid combinations in Nicotiana, sixteen of which have not been previously reported. In these studies little evidence was found supporting three theories proposed prior to this study to explain the cause of genetic tumors. Such tumors are always spontaneously initiated from within the plant, are produced in entire populations of certain crosses, are transmitted to the progeny by factors carried with the seed, and occur only when the plant is not growing rapidly. Of all the species tested, N. langsdorflii was involved in more tumor-forming combinations than any other single species. In two interspecific combinations tumor formation was associated with loss of chromosomes. Certain experimental data which cannot be explained by existing theories indicate that genetic tumors may have their origin in abnormal phytohormone relationships. Species comprise harmonious systems in which developmental processes, including phytohormone utilization, are integrated to produce normal growth. However, in certain interspecific combinations of Nicotiana, particularly with N. langsdorflii as one parent, there is a disturbance of this growth regulatory mechanism. It is hypothesized that this results, at maturity and occasionally in the seedling stage, in the inability of the plant to normally metabolize phytohormones which in turn, results ultimately in a stimulation of certain tissues to become meristematic and to grow abnormally without definite cell differentiation.